The present invention relates to shaft seals, and more particularly to hydraulic machines which embody hydrostatic shaft seals of the type wherein leak fluid can escape from the high-pressure side to the low-pressure side of the seal by way of a control clearance gap. Such seals are often utilized in coolant circulating pumps of nuclear reactor plants.
Heretofore known hydrostatic shaft seals function satisfactorily during normal operation of a pump or other machine which embodies the seal. However, when a pump which circulates coolant in a nuclear reactor plant is started, the temperature and/or pressure of conveyed fluid often varies at an unpredictable rate and is likely to adversely affect the action of hydrostatic seal. The reasons for such improper functioning of a hydrostatic seal during starting are as follows: In order to furnish a satisfactory sealing action, a hydrostatic shaft seal must maintain the stationary sealing ring which is mounted in the pump housing out of contact with the rotary sealing ring which shares the angular movement of the shaft and defines with the stationary ring a control clearance gap of desired width. The two rings will not contact each other if the difference between the pressure in the low-pressure compartment (which receives leak fluid by way of the gap) and the pressure in the high-pressure compartment (upstream of the gap) does not decrease below a predetermined minimum value. If the rings are permitted to contact each other, the surfaces which flank the gap undergo pronounced wear and the width of the gap increases when the pressure differential increases to the desired value. When the pump or the plant is started, the pressure differential is often so low that the rings come into actual contact with each other (at least one of the gap-defining rings in a hydrostatic shaft seal is movable axially toward and away from the other ring) and are subjected to extensive wear as a result of frictional engagement between the surfaces which flank the gap. Moreover, malfunctioning of one or more units in the plant often entails pronounced fluctuations of the temperature of conveyed fluid whereby the fluid causes thermally induced deformation of sealing rings with the result that the width of the control clearance gap deviates from an optimum width or from a range of acceptable widths.